1. Technical Field
The present invention relates to a communication system in which data is transmitted and received between a plurality of communication apparatuses. In particular, the present invention relates to a communication apparatus, a communication system and a communication method in which when a trouble occurs in a communication between communication apparatuses connected in series in a topology such as the ring topology or the daisy-chain topology, the function as a communication system can be automatically restored by the processing by each communication apparatus.
2. Description of Related Art
In recent years, in various fields, a system has been used in which a plurality of apparatuses are connected, a function is assigned to each apparatus and the apparatuses exchange data with each other and perform various processings in cooperation. For example, in the field of the vehicle-mounted LAN (local area network) disposed on vehicles, ECUs (electronic control units) each perform a processing specific thereto and are connected by a communication line to exchange data with each other, whereby various functions are implemented as a system.
With the specialization of the function of each apparatus and the increase in the number of functions that each apparatus can perform, the number and type of apparatuses connected to the communication line increases. Further, since various functions are expected as a system, it becomes necessary that the apparatuses share data and cooperate, so that the amount of transmitted data increases.
The increase in the amount of data transmitted and received by a communication line leads to a delay or loss of data due to a collision. A significant delay or loss of data is sometimes fatal to the driving assistance function such as brake control by the ECUs.
Therefore, a structure is typical in which the communication line is divided into a plurality of lines and the ECUs are connected to different lines, respectively. This is because wasteful use of the communication line can be suppressed by grouping ECUs that share data. In the field of the vehicle-mounted LAN, since the weight reduction of the system is particularly desired, a structure in which the number of communication lines is also small is desired. Since the communication lines are efficiently used to cope with the increase in the kind of ECUs, a structure is present in which ECUs are separately connected to communication lines of different communication speeds according to the kind of transmitted and received data. In these structures, the different communication lines are connected by relay apparatuses that control the transmission and reception of data.
Even when the ECUs are divided into a plurality of groups, the amount of data sent out to the communication lines cannot be reduced in a structure in which data necessary for controlling the ECUs are all transferred by the relay apparatuses. Accordingly, Japanese Patent Application Laid-Open No. 2008-22158 discloses a technology in which the data received from the ECUs is temporarily stored in a database and the ECUs read necessary data from the database as required and transmit it.